We observed less severe symptoms in patients with chronic idiopathic thrombocytopenic purpura. (ITP) than in patients with acute ITP with similar platelet counts. Thrombopoietin (TPO), soluble P-selectin, soluble P-selectin per platelet, and interleukin 6 (IL-6) were evaluated in children with ITP before treatment in 16 acute and 22 chronic cases and after treatment in 10 acute and chronic cases who received mega-dose methylprednisolone. The levels of IL-6, soluble P-selectin, soluble P-selectin per platelet, and platelet count were similar in acute and chronic ITP (P > 0.05) but TPO in acute ITP was higher than that of the patients with chronic ITP (P < 0.05). The posttreatment IL-6 and TPO declined (P < 0.05), but soluble P-selectin and platelet count increased (P < 0.05). Posttreatment soluble P-selectin per platelet levels were higher than the normal values (P < 0.05). These results suggest that IL-6, soluble P-selectin, and soluble P-selectin per platelet are not responsible for the milder symptoms in chronic than in acute ITP. Mega-dose methylprednisolone seems to keep the soluble P-selectin levels elevated.